1. Field of the Invention
The present invention relates to an apparatus for inspecting an electric component in an inverter circuit, and more particularly to an inspection apparatus for inspecting an electric component in an inverter circuit which converts a direct current into an alternating current while the electric component is being connected in an inverter circuit.
2. Description of the Related Art
Inverters have heretofore been used for turning on and off transistors based on control pulses supplied from a controller to convert a direct current into an alternating current for energizing a load such as a welding machine, an AC motor, or the like.
One typical inverter circuit comprises an electrolytic capacitor for smoothing a voltage, a plurality of transistors, and a plurality of diodes coupled in inverse-parallel connection to the respective transistors. These electric components have to be inspected immediately after they are assembled and subsequently at certain periodic time intervals because of deterioration due to usage. It has been customary to remove each electric component from the inverter circuit and inspect the removed electric component with a dedicated instrument to determine whether the electric component is of acceptable quality or not.
Heretofore, an electrolytic capacitor has been inspected as follows: The electrostatic capacitance and equivalent series resistance of the electrolytic capacitor are measured by an LCR meter, for example, and the electrolytic capacitor is determined as to its quality from the measured electrostatic capacitance and equivalent series resistance. In the inspection process using the LCR meter, a voltage across the electrolytic capacitor is measured while a small alternating current of several mA is passing through the electrolytic capacitor, and the electrostatic capacitance and equivalent series resistance of the electrolytic capacitor are calculated based on the measured voltage. The electrolytic capacitor is determined as to its quality by checking if the calculated electrostatic capacitance and equivalent series resistance fall in respective predetermined ranges or not.
Another inspection process is disclosed in Japanese laid-open patent publication No. 5-215800, for example. In the disclosed inspection process, an electrolytic capacitor to be inspected is charged through a resistor, and a charging time required from the time when the electrolytic capacitor starts being charged until it is charged to a predetermined voltage thereacross is measured. The measured charging time is compared with a reference time which is consumed until a voltage across a normal electrolytic capacitor reaches the predetermined voltage when the normal electrolytic capacitor is charged. If the measured charging time is shorter than the reference time, then the electrolytic capacitor which has been inspected is determined as being deteriorated.
Transistors for use in inverter circuits are required to be checked for saturated voltage vs. emitter current characteristics (V.sub.CE -I.sub.C characteristics) between the emitter and the collector at both a normal temperature and a predetermined elevated temperature in order to keep reliability of the inverter circuits. To determine whether a transistor of an inverter circuit is acceptable or not, it has been the conventional practice to disconnect the transistor from the inverter circuit, plot the V.sub.CE -I.sub.C characteristic curve of the transistor with a curve tracer, and check if the plotted V.sub.CE -I.sub.C characteristic curve falls within a predetermined range from a reference V.sub.CE -I.sub.C characteristic curve for the transistor. The V.sub.CE -I.sub.C characteristics are measured when the transistor junction is at a normal temperature, e.g., 25.degree. C., and a predetermined elevated temperature, e.g., 125.degree. C. The inspected transistor is accepted if both the V.sub.CE -I.sub.C characteristics measured at these temperatures fall within the predetermined range from reference V.sub.CE -I.sub.C characteristics.
According to the above conventional inspecting process of determining an electrolytic capacitor by calculating the electrostatic capacitance and equivalent series resistance of the electrolytic capacitor which are measured by the LCR meter, the voltage applied to the transistor is low as the current flowing therethrough is small. If an electrolytic capacitor used as a smoothing capacitor in an inverter circuit is inspected, then since conditions in which the electrolytic capacitor is inspected are greatly different from those in which it is actually used and the electrolytic capacitor is not inspected under the conditions in which it is actually used, the result of the inspecting process cannot be used as being obtained under the conditions in which it is actually used. Another problem is that the electrolytic capacitor to be tested has to be removed from the inverter circuit.
The equivalent series resistance of an electrolytic capacitor is considered as being important because it is responsible for the heating of the electrolytic capacitor and hence greatly affects the service life of the electrolytic capacitor. In the inspection procedure in which a measured charging time is compared with a reference time, the equivalent series resistance of an electrolytic capacitor being inspected cannot be measured or calculated as it is only possible in the inspection procedure to determine the electrostatic capacitance of the inspected electrolytic capacitor as being reduced a certain quantity from the electrostatic capacitance of the reference electrolytic capacitor.
The above conventional process for inspecting a transistor is problematic in that a transistor to be inspected has to be disconnected from the inverter circuit in question, and hence the inspecting process is tedious and time-consuming if the inverter circuit is in actual use. The process is time-consuming because the temperature needs to be increased from the normal temperature (25.degree. C.) to the elevated temperature (125.degree. C.). The process requires a dedicated instrument such as a curve tracer or the like, and involves a large number of steps for inspecting transistors on a mass-production line.
As described above, electric components for inverter circuits are individually inspected in the conventional inspecting processes. The conventional inspecting processes require a long period of time until the inspection of an electric component is finished because it is necessary to detach the electric component from the inverter circuit, inspect the electric component, and thereafter connect the electric component that is found acceptable back in the inverter circuit.